Laser device and optical amplifier

ABSTRACT

Laser light emission across a wide bandwidth emission spectrum is enabled in a laser device equipped with solid gain media. The laser device is equipped with: a resonator; a plurality of solid gain media, having fluorescent spectra that at least partially overlap with each other, provided within the resonator; and pumping means, for pumping the plurality of solid gain media. The entire fluorescent spectrum width of the plurality of solid gain media is greater than the fluorescent spectrum width of each solid gain medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser device and an opticalamplifier. Particularly, the present relates to a laser device and anoptical amplifier equipped with a plurality of solid gain media.

2. Description of the Related Art

Laser media having wide emission wavelength bands are favorably used inshort pulse lasers (mode locked lasers). Materials to which Yb(ytterbium) ions, which are rare earth ions, are added (as disclosed in“Femtosecond Yb:YAG laser using semiconductor saturable absorbers”, C.Honninger et al., Optics Letters, Vol. 20, No. 23, pp. 2402-2404, 1995),glass to which Nd (neodymium) ions are added (as disclosed in JapaneseUnexamined Patent Publication No. 6 (1994)-244486), and materials towhich Ti (titanium) ions, which are transition metal ions, are added (asdisclosed in U.S. Pat. No. 6,618,423) are known laser media having wideemission wavelength bands, particularly as solid laser media (solid gainmedia). Fibers to which Er (erbium) is added (as disclosed in U.S. Pat.No. 5,689,519) and the like are known as laser media having wideemission wavelength bands, as fiber lasers. Short pulse lasers of the p(pico) second and f (femto) second class require laser media havingextremely wide emission wavelength bands. Therefore, conventional shortpulse lasers of these classes were limited to employing laser media suchas those described above, that is, the wavelengths of the lasers werelimited to the infrared range. In addition, four level system Nd ions,which are advantageous in laser emission within the 1 μm band, havenarrow emission widths if used with YAG (Y₃Al₅O₁₂) as the base material,and cannot be utilized for short pulse lasers. Accordingly, glass basematerials are used with these ions, in order to widen the emissionwidth. However, glass has a low coefficient of thermal conduction, andtherefore there is a problem that Nd ion added glass solid lasers arenot suited for high output laser emission.

Various methods, such as those employing semiconductor lasers, pigmentlasers, Ti:Sapphire lasers, and OPO (Optical Parametric Oscillators)have been proposed to realize wavelength variable lasers. Particularly,many wavelength variable lasers employing semiconductor lasers arecurrently being developed. However, the emission ranges of these lasersare mostly within the infrared-near infrared range, and emission in onlyblue and violet color ranges have been realized within the visible lightspectrum. Ti:Sapphire lasers are commonly employed as wavelengthvariable lasers, but the emission wavelengths thereof are limited to thenear infrared range. These infrared-near infrared range emitting lasersare able to emit light within the visible light spectrum by utilizingSHG (Second Harmonic Generation). However, laser emission within thevisible light spectrum utilizing SHG cannot realize high efficiency norstable operation. In contrast, pigment lasers have a mean variablewavelength range of approximately 50 nm in the case that a singlepigment is employed, but it is possible to emit laser light within theultraviolet-infrared wavelength range by utilizing a plurality of typesof pigment. However, pigments have the problem that they deteriorate,and a drawback that pump light sources are expensive for shorterwavelengths. Meanwhile, OPO's are capable of covering a comparativelywide variable wavelength range, but there are problems, such as beamquality. Accordingly, there are advantages and drawbacks to currentlyavailable variable wavelength lasers, and industrial application thereofis difficult.

As described above, there is demand for a device which is capable ofstably emitting laser light across a wide spectral band ranging frominfrared through the visible light spectrum as either a short pulselaser or as a variable wavelength laser. In addition, there is similardemand for an optical amplifier for amplifying laser beams.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the foregoingcircumstances. It is an object of the present invention to provide alaser device capable of emitting laser light across a wide spectralband. It is another object of the present invention to provide anoptical amplifier capable of amplifying seed light beams having widewavelength bands.

A laser device of the present invention comprises:

a resonator;

a plurality of solid gain media, having fluorescent spectra that atleast partially overlap with each other, provided within the resonator;and

pumping means, for pumping the plurality of solid gain media; wherein:

the entire fluorescent spectrum width of the plurality of solid gainmedia is greater than the fluorescent spectrum width of each solid gainmedium.

Here, the “fluorescent spectrum width” refers to the full width at halfmaximum of a peak wavelength within the fluorescent spectrum band. Inthe case that a plurality of peak wavelengths exist, the fluorescentspectrum width is defined to be the widest full width at half maximumfrom among those of the peak wavelengths. The “entire fluorescentspectrum width” refers to the full width at half maximum of a peakwavelength within an entire fluorescent spectrum band, formed by thepartially overlapping fluorescent spectrum bands of the plurality ofsolid gain media. In the case that a plurality of peak wavelengthsexist, the entire fluorescent spectrum width is defined to be the fullwidth at half maximum of the peak wavelengths within a desired emissionwavelength range.

The pumping means may comprise a single pump light source or a pluralityof pump light sources, as long as it is capable of pumping the pluralityof solid gain media simultaneously.

It is desirable for the number and wavelength of fluorescent peaks ofeach of the plurality of solid gain media to be selected, and the solidgain media to be arranged such that the entire fluorescent spectrumwidth becomes a desired value. Further, it is desirable for theplurality of solid gain media to be arranged with the fluorescentintensities thereof being adjusted.

Alternatively, the wavelength, number, and power of pump light beamsemitted from the pumping means may be selected such that the entirefluorescent spectrum width becomes a desired value.

It is desirable for the plurality of solid gain media to be integrated.Here, the term “integrated” refers to a state in which the lightentrance and emission surfaces of the plurality of solid gain media arein contact with each other, or a state in which the plurality of solidgain media are in monolithic form. In addition, individually formedsolid gain media may be coupled by adhesive, optical contacts, or thelike. As a further alternative, each of the solid gain media may be of apolycrystalline structure, which are then stacked and sintered to beintegrated.

In the case that the plurality of solid gain media are integrated, it isdesirable for each of the plurality of solid gain media to be of apolycrystalline structure. In this case, it is desirable for each of theplurality of solid gain media to comprise rare earth ions added to abase material having one of a garnet type structure, a C rare earth typestructure, and a perovskite type structure. Further, it is desirable forthe plurality of solid gain media to have the same base material, towhich the same rare earth ions are added.

The laser device of the present invention may be employed favorably aseither a mode locked laser device or a variable wavelength laser device.

The laser device of the present invention may further comprise:

at least one wavelength converting element, for converting thewavelength of light beams emitted from the solid gain media; wherein:

wavelength converted light beams are output.

An optical amplifier of the present invention comprises:

a plurality of solid gain media, having fluorescent spectra that atleast partially overlap with each other; and

pumping means, for pumping the plurality of solid gain media; wherein:

the entire fluorescent spectrum width of the plurality of solid gainmedia is greater than the fluorescent spectrum width of each solid gainmedium. Note that it is desirable for the fluorescent peak wavelength ofeach of the plurality of solid gain media to be different. The“fluorescent spectrum width” and the “entire fluorescent spectrum width”are defined in the same manner as in the aforementioned laser device.

The pumping means of the optical amplifier may comprise a single pumplight source or a plurality of pump light sources, as long as it iscapable of pumping the plurality of solid gain media simultaneously.

It is desirable for the number and wavelength of fluorescent peaks ofeach of the plurality of solid gain media to be selected, and the solidgain media to be arranged such that the entire fluorescent spectrumwidth becomes a desired value. Further, it is desirable for theplurality of solid gain media to be arranged with the fluorescentintensities thereof being adjusted.

The wavelength, number, and power of pump light beams emitted from thepumping means may be selected such that the entire fluorescent spectrumwidth becomes a desired value.

It is desirable for the plurality of solid gain media to be integrated.The definition of the term “integrated” is the same as that for theaforementioned laser device.

In the case that the plurality of solid gain media are integrated, it isdesirable for each of the plurality of solid gain media to be of apolycrystalline structure. In this case, it is desirable for each of theplurality of solid gain media to comprise rare earth ions added to abase material having one of a garnet type structure, a C rare earth typestructure, and a perovskite type structure. Further, it is desirable forthe plurality of solid gain media to have the same base material, towhich the same rare earth ions are added.

The laser device of the present invention is equipped with the pluralityof solid gain media having fluorescent spectrum bands that at leastpartially overlap with each other, within the resonator. The entirefluorescent spectrum width of the plurality of solid gain media isgreater than the fluorescent spectrum width of any one solid gainmedium. Therefore, the combination of the solid gain media enablesemission of laser light over a wide wavelength range from visible lightto infrared light. Solid gain media are more stable than pigment,resulting in a laser device having high stability.

The optical amplifier of the present invention is equipped with theplurality of solid gain media having fluorescent spectrum bands that atleast partially overlap with each other. The entire fluorescent spectrumwidth of the plurality of solid gain media is greater than thefluorescent spectrum width of any one solid gain medium. Therefore, thecombination of the solid gain media enables emission of laser light overa wide wavelength range from visible light to infrared light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram that illustrates the construction of alaser device according to a first embodiment of the present invention.

FIG. 2 is a diagram that illustrates a modified laser media group.

FIG. 3 is a schematic diagram that illustrates the construction of awavelength variable laser device according to a second embodiment of thepresent invention.

FIG. 4 is a schematic diagram that illustrates the construction of anoptical amplifier according to a third embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the attached drawings.

FIG. 1 is a schematic diagram that illustrates the construction of alaser device 1 according to a first embodiment of the present invention.The laser device 1 comprises: a laser media group 5 and a pumping means2. The laser media group 5 is constituted by a resonator 10 and aplurality of solid gain media 51, 52, and 53, which are provided withinthe resonator 10 and have fluorescent spectrum bands that at leastpartially overlap with each other. The pumping means 2 is provided topump the plurality of solid gain media 51, 52, and 53.

The pumping means 2 comprises: a semiconductor laser 11, for emitting alaser beam L₁ as a pump light beam; and a focusing optical system 12,for leading the laser beam L₁ to the resonator 10.

The resonator 10 is constituted by a concave mirror 4 and asemiconductor saturable absorber mirror 7 (by BATOP Co., hereinafter,referred to as “SESAM”). A Brewster plate 6 is further provided withinthe resonator 10 as a polarization control means.

The laser media group 5 receives the laser bean L₁ as a pump light beam,and emits solid state laser light beams. The SESAM 7 causes mode lockedoperation, and a pulsed laser beam L₂ is output from the concave mirror4. The output laser beam L₂ is led to the exterior by a dichroic mirror3, which is provided between the focusing optical system 12 and theconcave mirror 4.

The plurality of solid gain media 51, 52, and 53 are characterized bythe entire fluorescent spectrum width thereof being greater than thefluorescent spectrum width of any one of the solid gain media 51, 52,and 53. A specific example will be described below.

The laser media group 5 may be constituted by arranging a Lu₃Ga₅O₁₂medium 51, a Gd₃Sc₂Al₃O₁₂ medium 52, and a Gd₃Sc₂Ga₃O₁₂ medium 53 inthis order, from the direction in which the laser beam L₁ enters. All ofthe media 51, 52, and 53 are of the garnet type, and Nd ions are addedto each of the media 51, 52, and 53. In this case, the fluorescent peakwavelengths of each of the media 51, 52, and 53 are 1062.3 nm, 1059.9nm, and 1061.2 nm, respectively. The fluorescent spectrum widths of eachof the media 51, 52, and 53 are 1.02 nm, 1.1 nm, and 1.6 nm,respectively. The Nd concentration within each of the laser media 51,52, and 53 is 1 at %, such that the amount of pumping light absorbed bythe laser medium 52 arranged in the center is greater than the amountsof pumping light absorbed by the laser media 51 and 53. The thicknessesof the laser media 51, 52, and 53 are 0.3 mm, 0.5 mm, and 0.5 mm,respectively. The laser media 51, 52, and 53 are arranged with intervalsof approximately 0.1 mm therebetween. In the case that the laser media51, 52, and 53 are pumped by a laser beam within the 810 nm band (thatis, in the case that the laser beam L₁ is within the 810 nm band), thefluorescent intensity of the maser medium 52 is greater than those ofthe laser media 51 and 53. The fluorescent peak wavelength of the threelaser media 51, 52, and 53 (1.06 μm) substantially matches thefluorescent peak wavelength of the laser medium 52 (1059.9 nm). Theentire spectrum width becomes approximately 3 nm, which is two to threetimes wider than the fluorescent spectrum widths of 1 nm to 1.6 nm ofthe individual laser media 51 through 53. Note that a coating thatfavorably transmits the laser beams L₁ and L₂ are provided on the facetsof the solid gain media 51 through 53.

The focusing optical system 12 comprises: an aspherical lens 13 with afocal distance f=8 mm; cylindrical lenses 14 and 15 with focal distancesf=-−7.7 and f=70, respectively; and an achromatic lens 16 having a focaldistance f=60. The laser beam L₁ emitted by the semiconductor laser 11is focused by the focusing optical system 12 such that the beam waist ofthe pump light beam is at the center of the Gd₃Sc₂Al₃O₁₂ medium 52. Notethat a laser having a wavelength within the 810 nm band and a beam widthof 100 μm is employed as the semiconductor laser 11. The focusingoptical system 12 forms a 50 μm×140 μm pump light beam waist within theGd₃Sc₂Al₃O₁₂ medium 52, which is arranged as the central laser medium.

A SESAM having an absorption rate of 0.7% with respect to a wavelengthof 1040 nm is employed as the SESAM 7. A concave mirror having atransmissivity of 1% with respect to the laser beam L₂ and a curvatureradius of 50 mm is employed as the concave mirror 4, and the resonatorlength is 50 mm in air. Note that this resonator structure enables abeam waist having a 1/e² diameter of 90 μm to be formed on the SESAM 7.The laser media group 5 is provided such that the distance between theSESAM 7 and the center of the Gd₃Sc₂Al₃O₁₂ medium 52 is approximately 15mm. At this position, the 1/e² diameter of the beam becomesapproximately 240 μm.

Garnet type laser media to which Nd was added were used in the presentlaser device. The emission light wavelength band of the present laserdevice is wider than that of an Nd:YAG laser, for example. Therefore,shortening of pulses becomes possible.

Specifically, in the case that the present laser device is employed, apulse light beam having a mean output of 100 mW, a pulse width of 3psec, and a cyclic frequency of 3 GHz was obtained with a pumping powerof 2 W.

Three solid gain media were employed in the first embodiment. However,the number of solid gain media is not limited to three. Pulse widths canbe freely controlled, by controlling the emission spectrum widths. Thethicknesses of the laser media were adjusted to adjust the fluorescentintensity in the present embodiment. Alternatively, the concentration ofthe added rare earth ion may be adjusted. Any rare earth ion may beapplied, and if rare earth ions such as Pr, Sm, Eu, Tb, Dy, Ho, and Erare employed, short pulse lasers within the visible light spectrum canbe realized.

Note that in the first embodiment, the solid gain media of the lasermedia group were provided with intervals of 0.1 mm therebetween.Alternatively, the plurality of solid gain media 51, 52, and 53 may beintegrated such that their light entry and light emission surfaces arein contact with each other, as illustrated in the side view of a lasermedia group 5′ of FIG. 2. Individually formed solid gain media may becoupled by adhesive, optical contacts, or the like. As a furtheralternative, each of the solid gain media may be of a polycrystallinestructure, which are then stacked and sintered to be integrated.

Crystalline structure materials other than garnet type materials may beemployed as the base material for the laser media. Other than garnet,perovskite and C-rare earth crystalline structure materials arepreferred. Further, it is desirable for the plurality of solid gainmedia to have the same base material, to which the same rare earth ionsare added. In the case that the same base material and the same rareearth ions are employed, there are no large differences in stimulatedemission cross sectional areas and fluorescent lifetimes. Therefore,matching of threshold values is facilitated, and light emission form allof the laser media can be effectively utilized.

A resonator having a Fabry-Perot type structure was described in thefirst embodiment. However, any type of structure may be adopted for theresonator, such as the Z-type structure and the bowtie type structure.

At least one wavelength converting element may be provided either withinor outside the resonator of the laser device of the first embodiment,and wavelength converted light beams may be obtained.

The present invention is not limited to being applied to mode lockedlasers. The present invention functions effectively as a variablewavelength laser if provided with a wavelength controlling element, andalso as an optical amplifier.

FIG. 3 is a schematic diagram that illustrates the construction of awavelength variable laser device 20 according to a second embodiment ofthe present invention. The variable wavelength laser device 20 isequipped with the same laser media group 5 as the laser device 1 of thefirst embodiment. A facet of one of the solid gain media (a facet 51 atoward the pump light beam incident side of the solid gain medium 51,which is arranged closest to the pump light source) and a concave outputmirror 22 constitute a resonator 25. The pumping means 2 is the same asthat of the laser device 1 of the first embodiment.

A concave output mirror having a transmissivity of 1% with respect towavelengths within a range of 1060 nm to 1070 nm is employed as theconcave output mirror 22. Further, a birefringence filter 26 thatfunctions as a wavelength modulating element is provided within theresonator 25. Linearly polarized light beams can be emitted, byproviding the birefringence filter 26 at Brewster angles with respect tothe optical axis of the resonator 25.

The variable wavelength laser device 20 is capable of emitting laserlight beams L₂ within a wavelength range of 1062 nm to 1066 nm, by theoperation of the birefringence filter 26.

FIG. 4 is a schematic diagram that illustrates the construction of anoptical amplifier 30 according to a third embodiment of the presentinvention. The optical amplifier 30 is a solid state regenerative laseramplifier that employs the same laser media group 5 as the laser device1 of the first embodiment.

An Nd:YVO₄ mode synchronized oscillator 40 (central wavelength=1064 nm)is prepared separately as a seed laser. The pulse width of a short pulselaser beam Ls (seed light beam) is stretched by a diffraction grating(not shown). Thereafter, the laser beam Ls is introduced into theregenerative amplifier 30 via a polarizing element 41. A combination 42of an electro-optic device and a ¼ wavelength plate sets the voltage tobe applied to the electro-optic device to be zero. Thereby, a ¼wavelength phase difference is imparted to the laser beam Ls, which isthen input to the regenerative amplifier 30. During the second pass,another ¼ wavelength phase difference is imparted, and the photon isconfined within a resonator. A half wavelength voltage is applied to theelectro-optic device, or the voltage is turned OFF, to ultimately obtainthe photon (for details of this operation, refer to Kochner, Solid StateLaser Engineering, Vol. 4, p. 541).

A pumping means 34 comprising a semiconductor laser 32 and an opticalsystem 33 emits a pumping light beam Le. The light beam enters the solidgain media group 5 via a concave mirror 39. The seed light beam Ls isamplified, by reciprocating between reflective mirrors 36 and 37 viaconcave mirrors 38 and 39. Ultimately, a pulse light beam Lp, of whichthe amplified pulse has been compressed, is output from the reflectivemirror 36.

In the third embodiment, a regenerative amplifier 30 has been describedas an example of a solid state laser amplifier. However, the laseramplifier of the present invention is not limited to regenerativeamplifiers. The present invention may be applied to single pass typelaser amplifiers and multi pass type laser amplifiers, such as doublepass type laser amplifiers and 4 pass type laser amplifiers.

1-11. (canceled)
 12. An optical amplifier, comprising: a plurality ofsolid gain media, having fluorescent spectra that at least partiallyoverlap with each other; and pumping means, for pumping the plurality ofsolid gain media; wherein: the entire fluorescent spectrum width of theplurality of solid gain media is greater than the fluorescent spectrumwidth of each solid gain medium.
 13. An optical amplifier as defined inclaim 12, wherein: the number and wavelength of fluorescent peaks ofeach of the plurality of solid gain media are selected, and the solidgain media are arranged such that the entire fluorescent spectrumbandwidth becomes a desired value.
 14. An optical amplifier as definedin claim 13, wherein: the plurality of solid gain media are arrangedwith the fluorescent intensities thereof being adjusted.
 15. An opticalamplifier as defined in claim 12, wherein: the wavelength, number, andpower of pump light beams emitted from the pumping means are selectedsuch that the entire fluorescent spectrum bandwidth becomes a desiredvalue.
 16. An optical amplifier as defined in claim 12, wherein: theplurality of solid gain media are integrated.
 17. An optical amplifieras defined in claim 16, wherein: each of the plurality of solid gainmedia is of a polycrystalline structure.
 18. An optical amplifier asdefined in claim 17, wherein: each of the plurality of solid gain mediacomprises rare earth ions added to a base material having one of agarnet type structure, a C rare earth type structure, and a perovskitetype structure.
 19. An optical amplifier as defined in claim 18,wherein: the plurality of solid gain media are of the same basematerial, to which the same rare earth ions are added.